1. Field of the Invention
The present invention is a process of forming optical components, such as lenses, by heating and pressing an optical material in a mold while controlling the temperature and pressure as a function of time.
2. Description of Related Art
It has been conventional to shape optical components by grinding and polishing optical materials such as crown glass, flint glass, etc., so as to form, for example, lenses. In recent years relatively small diameter aspherical surface lenses made from optical glass have been press-formed in molds. Optical material in pillar, cylinder or ball shapes are placed on the lower portion of a two-section mold that is usually concave on its upper surface, with the concavity in the shape of a lens lower surface. An upper mold is provided having a concavity being in the shape of the lens upper surface. A heating device is provided to heat the optical material and a drive mechanism is provided to press the two mold sections together. The optical glass placed on the lower mold section is heated until it is soft and deformable under pressure. This is referred to as the plasticization temperature or plastic state. When molding glass, the glass is heated above its transition point, which is the minimum temperature necessary for molding. With the plasticization temperature maintained, either or both of the mold sections may be moved, but usually the upper mold is lowered so as to apply pressure on the optical material. The shaped optical material is then gradually cooled and pressure is applied to it between the upper and lower mold sections. The rate of temperature drop is controlled so that cooling is at a slower rate than it would be if the mold apparatus with enclosed optical component were exposed to room or ambient temperature and natural cooling occurred. After the controlled temperature drop, the mold apparatus with enclosed optical material (now a formed optical component such as a lens) is then naturally cooled by exposure to ambient or room temperature. After the optical component has cooled, it is removed by separating the mold sections.
However, sometimes air is trapped between the optical material and a mold section, usually as a result of the initial shape of the optical material that is placed in the mold including a flat surface and the mold section having a concave surface, resulting in a gap when the mold section first contacts the optical material. Due to air being trapped in such a gap, the shape of the mold will not accurately be transferred to the optical component being molded. The problem of trapping air is not pronounced when using an optical material that is initially ball-shaped, because of the outward spread of the optical material from the initial point of contact with the mold section. However, the problem is known to occur where the staring optical material is pillar-shaped or cylindrical in shape. To prevent this defect, the trapped air must be expelled during the molding process. Various prior art methods have been proposed for expulsion of the trapped gas in the case where the starting shape of the optical material is pillar-shaped or cylindrical in shape.
One such process for removing trapped gas releases the pressure from the mold sections once during the initial pressing step. The pressure relief expels gas from the mold. After the gas is expelled, the temperature is gradually lowered while the prior formation pressure (herein termed the molding pressure) is re-applied. The formed lens is then allowed to cool naturally with the only pressure on the lens being that of the weight of the upper mold section. This process is disclosed in Japanese Laid Open Patent Applications H02-102133 (1990), H02-252629 (1990), and H069228 (1994).
A second process is essentially the same as the first except that, during the gradual temperature reduction, the pressure is again relieved from the mold to again expel gas from the mold. This process is disclosed in Japanese Laid Open Patent Publication No. 3-69520 (1991).
A third process relieves pressure between the mold sections during the gradual temperature reduction in order to expel gas from the mold while the weight of the upper mold section is maintained on the lens during the natural cooling period. This process is disclosed in Japanese Laid Open Patent Publication H05-221664 (1993).